Wheat–Pea Intercropping Responds to Nitrogen Fertilization and Maintains Yield Under Agroforestry in Central Italy

Agriculture must shift to sustainable practices that support mitigation and adaptation, with crop diversification highlighted as a key adaptive practice. A field experiment was conducted in central Italy to study forage and grain production of wheat (Triticum aestivum L.) and pea (Pisum sativum L.) intercropping (IC) comparing three nitrogen fertilizations (NF) (i.e., 0, 70, and 140 kg ha−1) and two cropping systems (CS) (i.e., arable (AR) and silvo-arable (SIAR)) for two consecutive cropping seasons (2023–2024 and 2024–2025). Forage and grain production were determined at flowering and at maturity to identify temporal trends in resource use. Overall, the results demonstrated that poplar presence did not significantly impact IC productivity, as forage biomass at flowering was 5.00 t ha−1 in AR and 4.77 t ha−1 in SIAR in 2024, and 3.20 t ha−1 in both cropping systems in 2025. Moreover, NF was the main driver of both forage and grain production, without significant interaction with the CS, and a moderate N rate (i.e., 70 kg ha−1) was the most efficient, ensuring both wheat and pea productivity. The absence of a yield penalty in the IC in the SIAR supports the agroecological value of integrating annual intercrops with tree components.